Hydraulic fracturing is an often used technique to increase the efficiency and productivity of oil and gas wells. Overly simplified, the process involves the introduction of a water-based, oil-base or emulsion fracturing fluid into the well and the use of fluid pressure to fracture and crack the well stratum. The cracks allow the oil and gas to flow more freely from the stratum and thereby increase production rates in an efficient manner.
There are many detailed techniques involved in well fracturing, but one of the most important is the use of a solid “proppant” to keep the stratum cracks open as oil, gas, water and other fluids found in well flow through those cracks. The proppant is carried into the well with the fracturing fluid which itself may contain a variety of viscosity enhancers, gelation agents, surfactants, etc. These additives also enhance the ability of the fracturing fluid to carry proppant to the desired stratum depth and location. The fracturing fluid for a particular well may or may not use the same formulation for each depth in the stratum.
Water produced during oil and gas operations constitutes the industry's most prolific by-product. By volume, water production represents approximately 98 percent of the non-energy related fluids produced from oil and gas operations, yielding approximately 14 billion barrels of water annually.
According to the American Petroleum Institute (API), more than 18 billion barrels of waste fluids from oil and gas production are generated annually in the United States. Such waste materials are often dissolved in subterranean water with a ratio of produced water to oil of about 10 barrels of produced water per barrel of oil. This contaminated water may include various ionic contaminants that include salt, hydrocarbons, heavy metals (e.g., zinc, lead, manganese, boron, copper, mercury, chromium, arsenic, strontium and aluminum), corrosive acids or bases from dissolved sulfides and sulfates, scale (e.g., insoluble barium, calcium and strontium compounds), naturally-occurring radionuclides (e.g., uranium, thorium, cadmium, radium, lead-210 and decay products thereof) often referred to as Naturally Occurring Radioactive Materials (NORMS), sludge (oily, loose material often containing silica and barium compounds) and dissolved radon gas. In general, the produced waters are re-injected into deep wells or discharged into non-potable coastal waters. Excluding trucking costs, waste water disposal can cost as much as $2 per barrel. Such costs must be factored into the overall economics of a gas field.
The NORMS contaminants are a matter of particular interest. Oil and gas NORM is created in the production process, when produced fluids from reservoirs carry sulfates up to the surface of the Earth's crust. Barium, Calcium and Strontium sulfates are larger compounds, and the smaller atoms, such as Radium 226 and Radium 228 can fit into the empty spaces of the compound and be carried through the produced fluids. As the fluids approach the surface, changes in the temperature and pressure cause the Barium, Calcium, Strontium and Radium sulfates to precipitate out of solution and form scale on the inside, or on occasion, the outside of the completion string and/or casings. The use of the completion string of tubular pipes in the production process that are NORM-contaminated does not cause a health hazard if the scale is inside the tubular string and the tubular string remain downhole. Enhanced concentrations of the radium 226 and 228 and the degradation products (such as Lead 210) may also occur in sludge that accumulates in oilfield pits, tanks and lagoons. Radon gas in the natural gas streams also concentrate as NORM in gas processing activities. Radon decays to Lead 210, then to Bismuth 210, Polonium 210 and stabilizes with Lead 206. Radon decay elements occur as a shiny film on the inner surface of inlet lines, treating units, pumps and valves associated with propylene, ethane and propane processing systems.
The contaminated water produced from a well should be reused or treated to remove the contaminants, especially the heavy metals. Oil wells are not, however, typically located next to substantial water treatment facilities. The contaminated water must be captured and transported to treatment facilities or portable facilities must be brought to the well. Exemplary systems have included packed beds of activated charcoal for the removal of organic compounds, permanent or portable ion exchange columns, electrodialysis and similar forms of membrane separation, freeze/thaw separation and spray evaporation, and combinations of these. All of these options are relatively costly with the water volumes produced from a production well.
Some type of in situ treatment could be potentially very helpful to supplement or, in some instances, replace surface-based purification treatments. Pointedly, it would be desirable if a proppant composition that is passed into the fractured well stratum could provide both a crack propping function as well as an ability to remove at least some portion of the ionic and dissolved contaminants before they were produced to the surface.
One publication that suggests the use of a dual function proppant is Tanguay et al. WO 2010/049467. The proppant described in this published application has a polycarbodiimide or polyurethane coating that optionally contains organic compounds, microorganisms and petroleum processing catalysts. As the coating dissolves over a period of 4 hours, the compounds, microorganisms or catalyst are slowly released into the crude oil.
U.S. Pat. No. 6,528,157 describes a coated proppant that includes a fibrous material extending outwardly from the proppant coating. This fuzzy proppant is said to be useful for acting as a physical screen to prevent the backward flow of sand, proppants or other particles from the fractured stratum. The fibers may be any of various kinds of commercially available short fibers such as milled glass fibers, milled ceramic fibers, milled carbon fibers and synthetic fibers having a softening point above typical starting sand temperature for coating, e.g., at least about 200° F. so as to not degrade, soften or agglomerate.
U.S. Pat. No. 7,754,659 teaches the addition of magnetic particles on the outside of a proppant substrate for the purpose of enhancing the flow-back resistance of the coated proppant from the fractured subterranean stratum.